Increase contrast for the currently selected session

### Feature summary

Make the currently selected session clearly distinguishable with a higher-contrast background and border.

### What problem are you trying to solve?

The current selected-session state in the session list is too subtle. With eye-friendly monitor settings commonly used by developers who spend all day looking at screens—lower brightness, warmer color temperature, reduced contrast, night-light modes, or similar calibration—it is difficult to tell which session is currently active.

This creates repeated visual friction when working with many parallel sessions: I have to stop and carefully scan the sidebar before I can confirm which session I am viewing. The selection state should remain obvious across supported themes and display configurations.

### Proposed solution

Use a visibly stronger selected state for the current session row:

Apply a filled, higher-contrast background using the active theme's accent or selection token.
Add a clear border or leading indicator in the same accent color used for the focused composer/text-box border.
Preserve readable text and icon contrast in light and dark themes.
Keep hover, unread, running, and selected states visually distinct.
Aim for at least 3:1 non-text contrast between the selected state and adjacent session rows.

For the current GitHub theme, a green-tinted selected background matching the focused text-box border would work well. The exact color should remain theme-aware rather than hard-coded.

### Workflow impact

This would help developers managing many concurrent sessions, especially people using low-brightness, warm-color, reduced-contrast, night-light, or other eye-comfort monitor settings during long workdays. It would make session switching faster, reduce accidental work in the wrong session, and improve basic visual accessibility.

### Installation context

GitHub Copilot desktop app on Windows; applies to the session sidebar across platforms.

### Additional context

The key requirement is persistent, immediately recognizable contrast for the active session—not only a subtle hover effect or small icon. Ideally the selected state should be tested across every built-in theme and under reduced-contrast display conditions.

首先定位会话侧边栏中选中行的样式，以及聚焦的 composer 或文本框边框所使用的主题 token。比较内置浅色和深色主题中的选中、悬停、未读和运行中状态，然后验证选中的会话在视觉上清晰可辨，同时文本和图标仍保持可读性。

由索引模型根据 Issue 内容生成。